Type the form argument rule with the classes that have an element

FormArgument itself declares neither ufl_function_space nor ufl_element, so
annotating the handler with it fails mypy.  Its subclasses Argument and
Coefficient are what actually reach the rule, and both carry them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: pbrubeck

ufl/algorithms/apply_function_pullbacks.py

@@ -11,7 +11,7 @@
 from functools import singledispatchmethod
 
 from ufl.algorithms.map_integrands import map_integrands
-from ufl.classes import Expr, FormArgument, Interpolate, ReferenceValue
+from ufl.classes import Argument, Coefficient, Expr, FormArgument, Interpolate, ReferenceValue
 from ufl.corealg.dag_traverser import DAGTraverser
 from ufl.domain import extract_unique_domain
 from ufl.form import BaseForm
@@ -39,7 +39,7 @@ def _(self, o: Expr) -> Expr:
         return self.reuse_if_untouched(o)
 
     @process.register(FormArgument)
-    def _(self, o: FormArgument) -> Expr:
+    def _(self, o: Argument | Coefficient) -> Expr:
         """Represent 0-derivatives of a form argument on the reference element."""
         r = ReferenceValue(o)
         space = o.ufl_function_space()